1. Field of the Invention
This invention relates to the field of snubbers such as used in the nuclear power industry, and more particularly to methods and apparatus for in-place testing of such snubbers.
2. Prior Art
In the United States at the present time, the approximately 130 nuclear power plants operating or under construction utilize hydraulic and mechanical snubbers as a means of providing on-demand rigid struts to protect equipment and piping from potential seismic and accident transients while allowing for free movement of the piping and equipment during thermal expansion. Hydraulic snubbers are generally in the form of a hydraulic cylinder and piston assembly, with the cylinder being attached to a reaction wall or other support structure and the piston rod being attached to the equipment to be protected. The two chambers on the opposite sides of the piston are in communication with each other through control valves which remain open under normal conditions to allow the position of the piston within the cylinder to freely drift in accordance with the differential expansion encountered, but which valves effectively close or restrict under more dynamic conditions to relatively rigidly couple the equipment to the support structure. This rigid coupling eliminates devastating vibrations and resonances. Snubbers of this general type are manufactured by Paul-Munroe Hydraulics Inc., assignee of the present invention.
Snubber sizes range from providing resistance loads of as little as several hundred pounds to in excess of two million pounds. The corresponding weight of the larger units are in the one to one and one-half ton range. The snubbers in Boiling Water Reactor plants (approximately 40 BWRs operating or under construction in the U.S.) generally do not exceed 150,000 pounds in capacity as opposed to the Pressurized Water Reactor plants (approximately 90 PWRs operating or under construction in the U.S. ) utilizing up to the largest snubbers for protection of their reactor coolant pumps and steam generators.
In 1980 and again in 1981, the U.S. Nuclear Regulatory Commission issued generic letters to the nuclear power industry requesting performance of routine in-service inspection on their snubbers. This was to ensure that the snubbers, which perform a safety related function in the power plants, were in good repair. This requirement by the NRC has caused the nuclear power industry to seek cost effective means of testing snubbers.
Until approximately three years ago, the only method available to the nuclear power industry for in-service inspection testing was with a test bench. A test bench tests the snubber directly by applying an external load to the snubber causing movement of the snubber piston which results in pressurization of the unit. This application of an external load tests the snubber in a manner analogous to its installed condition. While the test bench is a proven and reliable technology it requires the unit to be removed from the plant for testing. This is extremely expensive in terms of removal and reinstallation costs, outage costs, radiation exposure costs, snubber decontamination costs and testing costs. In some cases it is not possible to remove the snubbers without cutting out piping and concrete or removing equipment.
In an attempt to find a solution to the problem of removing the snubber, test machines were developed in the last three years to allow testing of the snubbers in a partially installed condition. Although termed in-place testing, this in fact is a misnomer because removal of the rod end pin is required to perform the test. The in-place test is an indirect test of the snubber because the unit is pressurized by introducing snubber hydraulic fluid through fill and bleed plugs (26 and 27 in FIGS. 1 and 11) to initiate movement of the rod which simulates external loading.
The snubber is tested for the same four tests as with the test bench: lockup and bleed tests to verify the performance of the control valves, and breakaway and drag to verify the performance of the seals. Current in-place testing techniques require performance of the breakaway and drag tests while the rod is being stroked, with the lockup and bleed tests being performed with the rod either fully extended or retracted.
Although there are some reduced radiation exposure, removal and reinstallation, outage, decontamination, and testing costs, these costs still remain relatively high, primarily because of the pin removal requirement. In fact in approximately 80% of the cases, even with the rod end pin removed, the test cannot be performed because the rod end pivot cannot clear the supporting equipment. In these cases the snubber still has to be removed and tested on a test bench. Because of the problems associated with the partially installed configuration, existing in-place test technology has not provided the solution that the nuclear industry has been seeking.